Secondary air control for furnaces



Oct. 11, 1938. PQo. HENDEnsoN SECONDARY AIR CONTROL FOR FURNAC-ES Filed May 4, `19257 2 Sheets-Sheet l Hilo um? Oct. 1l, 1938. P. o. HENDERSON SECONDARY AIR CONTROL FOR FURNACES 2 Sheets-Sheet 2 Filed nay 4, 1937 Patented Oct. 11, 1938 UNITED STATES PATENT oFFlcl-z SECONDARY AIB CONTROL FOB VII'URNACS Philip 0. Henderson, Washington, D. C., asli'gnor oi' one-third to John ODea and one-third to B. Earl Wenger, both of Washington, D. C.

Anuman my 4, 1937. sum N. 140,150

12 (Cl. 15s-za) My invention relates to control means for secondary air supplied to furnaces, especiallyfurnaces provided with burners operating on oil, gas or pulverulent solid fuel. s

'I'he invention provides a plentifulv and regulated supply of secondary air and incorporates means for preventing emission of products of combustion through the secondary ir supply opening whenever for any reason back pressures develop in the flrebox. In one form of embodiment the invention incorporates means for makingvthe rate of supply of secondary air responsive to any suitable high-low flame control used with the furnace, and a preferred form of embodi- 15 ment contemplates the use of closure means in the secondary air supply passage responsive to back pressures to close the passage and responsive to the high-low flame control partially to close the passage. s ,y v In burners for fluid and analogous fuel, primary air is of course mixed with the fuel supplied to the burner, and to support complete and most eiiicient combustion generous quantities of secondary air are also required. This secondary air is commonly admitted in part adjacent the burner, but more particularly and in larger quantites below the burner. For example, in furnaces which have been converted vfrom coal burners to oil or gas burners, the ash removal door in the lowermost part of the front Wall below the level of the grate is kept open, or a shutter in it is kept open, to admit most of Vthe secondary air required, the remainder being y supplied through an opening in the furnace wall around or adjacent the burner nozzle.

It sometimes happens that downdrafts occur during burner operation, and it rather frequently happens that backfires take place in the ilrebox, producing sudden and sometimes violent increases in pressure in the flrebox and flue and of course at the secondary air inlet. Both downdrafts and backflres thus have the eil'ect of reversing the ow at the secondary air inlet, so that the gaseous contents of the rebox tend to issue through that inlet into the room or building containing the furnace. This is maniftly objectionable, particularly in the case of backflres, becausein such cases the gases discharged into the room are generally the products of incomplete combustion, frequently high in carbon monoxide and almost invariably heavily soot-laden-- An object of the vpresent invention is to provide means associated with the secondary air inlet to prevent such discharges without in any way interfering with the function of the inletto admit secondary air during normal operation of the furnace and burner.

A related object of the invention is to relieve the pressures against the described closure means when backflres of unusual violence occur, so that 5 damage to the parts is avoided.

The high-low flame control usually employed with oil and gas burners, particularly in the larger installations, comprises means responsive to boiler or other temperatures or pressures for 10 regulating the flame size so that overheating is prevented and so that during periods when high heat generation is not required the boiler will be fired by a flame of reduced size. 'Ihe present invention cooperates with such a control by regu- 15 lating the rate of supply of secondary air in accordance .with flame size, so that the required quantities of secondary air are supplied for the various flamesizes and particularly so that excess'ive quantities of air are not admitted to the 20 nrebox when the flame is low. Thus, undesired cooling of the ilrebox is prevented.

Other objects of the invention are to provide means for the purposes specified which shall be simple, inexpensive, reliable and foolproof in opg5 eration, and entirely devoid of upkeep cost. Further objects and advantages will appear from the description hereinafter of a preferred embodiment of the invention.

In the accompanying drawings, which illustrate 30 a preferred embodiment which has been found reliable and satisfactory in actual use, and which hastherefore been selected as illustrative of the principles of the invention andnot to limit the scope of the invention to the details disclosed.

Figure .l is a vertical sectional view through a portion of a furnace front wall with which an embodiment of the invention is associated;

Fig. 2 is a front elevational view of the parts shown by l'lg. 1; 40

Fig. 3 isa. horizontal sectional view through the secondary air inlet duct, taken on the line 3 3 of Fig. l;

Fig. 4 is a vertical section through the duct at itsV inlet end, taken on the line 4 4 of Fig. 3; 45

Figs. 5 and 6 are detail views showing the latch by which the safety doors are yieldingly held in closed position on the shutters; and

Figs. 7 and 8 are detail sectional views showing the connecting duct or riser by which the two 50 secondary air inlet openings are placed in oommunication.

In the drawings, i designates a furnace, shown as a comparatively large type used to ilre a steam boiler, but intended to be typical of any kind 55 of heating furnace. The furnace is provided with a wall 2, conveniently the front wall, having an opening throughwhich extends the nozzle 4 of a burner`5 operating on oil, gas, pulieerized coal or the like. Preferably below the furnace, and below the grate 6 thereof, the wall 2 contains an additional opening 1, which constitutes the main inlet for secondary air, certain quantities of rsecondary air being admitted through the opening 3. It is kto be understood that the foregoing arrangement is old and conventional and has proved unsatisfactory in use because downdrafts and backres in the firebox cause products of combustion to be discharged through the openings 3 'and 1 into the room containing the furnacei lThe structure which will now be described comprises the arrangement proposed by the present invention to prevent these discharges from the firebox into the room containing the furnace.

Fastened to the furnace wall 2 so ias to surroundwthe opening 1 isa secondary air inlet duct 8, conveniently in the form of a sheet metal box housing open at its front or inlet end Within the duct 8 I provide closure rneans for the duct, normally open to admit air from the inlet end 9 to the discharge end IIJ adjacent the furnace wall, as shown by the solid line arrows in Fig. 3, butadapted to close in response to back pressures tending to cause gas to flow from the rebox 'through the duct inthe direction of the dotted line arrows of Fig. 3. This closure means comprises shutters II mounted relatively near the inlet end'9 of the duct, as by being hinged on the mounting piate I2 ar,- ranged vvertically in the yduct at'about its center, so that the shutters may swing outwardly in response to back pressure and close against the duct side walls, so that the duct is more or less completely closed by the shutters II or by the shutters II and their mounting plate I2.

1"ne shutters II are normally in the open position shown by the solid lines in Fig. 3. In this position they rest against the adjustable stops I3 mounted on a wall of the doctfand they are biased to this open position yby the incoming current of air or by some positive biasing means. The turning axis of each shutter may be inined slightly, or light springs may be used, if positive biasing is desired. In either event, in

open position the, shutters make obtuse angles with the mounting plate I2, so that they form the converging sides of a pocket opening in the direction from which reverse new will move when back pressure occurs, all as shown in Fig. 3. y

If the closure means kcomprised only the shutters II, a considerable amount of the gaseous contents of the firebox would pass out of the duct before` the shutters' II would move to their dotted line position to closethev duct, and in the oase of incomplete combustion a greatoeal of soot would be emitted to the room.

In order to make the closure means sumciently rsensitive and quick acting to close before much yor any of the gaseous contents of the firebox pass out of the duct, I include as part of the closure means a second pair of shutters I4,

` arranged relatively near the discharge end I0 of the duct. These shutters are pivoted on hinges I5 lspaced apart transversely of the duct and closed, either under the infiuence or incoming air or by some positive biasing means, against the adjustable stops I6 which are ingeneral like vthe stops I3. In their normal, open position, shown by full lines in Fig. 3, the shutters I4 forni a pocket with its walls converging in the direction of its bottom, and its bottom opening between the pivots I5. In their closed position the shutters I4 engage the side walls of the duct, and it will be understood that the shutters I4 swing on their pivots I5 to assume closed position as soon as back pressure occurs in the duct. Having moved to closed position, the back pressure is mrected-through the partially closed duct, between .the pivots I5 into the pocket formed by the still open shutters These shutters II thereupon promptiymove to closed position,vas shown by the dotted lines in Fig. ,3, and the duct becomes completely closed. It is to be observedthat the duct and lower part of the flrebox are normally filled with fresh air. When reverse ow occurs it is this fresh air which actuates the shutters I4 and in prompt succession the shutters II. Flow past the several shutter means and out through the inlet end 9 of the duct is thus of fresh air only', so that soot and other products of combustion are not discharged into the room containing the furnace. This feature of the invention I regard as of great importance. v

It will be appreciated that with the duct closed as has been explained the gases in the flrebox will normally be released through the smoke flue. However, it sometimes happensgin the case of comparatively violent expiosions at the borner or elsewhere in the firebox, that sufficiently high pressures are generated to disrupt the parts. To prevent damage in such cases I provide safety doors I1 vin the shutters and similar safety doors I8 in the shutters II. In a preferred form of embodiment the shutters take lthe form of open frames and the safety doors are imperferate plates spring pressed to close these open frames. The safety doors are hinged to the shutters opposite the ends of the shutters which are pivoted in the duct. Opposite their hinges the safety doors are latched to the shutters preferably by an vadjustable means like that shown a't I9 in Fig. 6, so that the safety doors may be made openable by an adjustable, predetermined pressure applied to them. The means I9, as shown,A comprises an adjusting screw carrying a spring latch element bearing against the free end of the safety door and holding the door against its shutter with a pressure determined by the adjustment of the screw in its mounting. Obviously, other types of Alatch might be substituted. Y

In the case of a backfire of predetermined violence, the shutters swing to closed position and the safety doors swing free of the'latches and lie against the side walls of the duct, as shown in the case of the doors'v I1 in dotted lines in Fig. 3. The doors I8 open in the same manner.

As has been explained, it is customary to admit a portion of the secondary air through the opening 3 or elsewhere in the immediate vicinity of the flame. Ordinarily, this secondary air is admitted directly from the room and passes between the fan housing 50 and burner front plate 5 I, and thence through a jacket within the burner nozzle 4. In order to prevent emission of gas from the flrebox through this secondary air path and into the room containing the furnace, I provide a duct connecting this secondary air inletr with the duct 8, so that secondary air taken in at the nozzle 4 must enter the inlet end 9 of duct 8 and pass through duct 8. This arrangement requires reverse flow Y through the upper 15 and extending to `.the burner I.

vreaches the shutters i'l.

inlet to tbe directed into the duct 8, where it is trapped by the closing of the shutters, so that gases passing out' of the furnace through the upper inlet are not admitted to the room containing the furnace.` Y

A suitable'- arrangement to accomplishV this purpose consists in a connecting duct2l rising from an opening in the top wall of the duct l n An inverted U-shaped iiange 2| nils thefspace between the fan housing 50 and the burner front plate Si, and thisiiange is connected' to the riser duct 28 so that the upper secondary air inlet is isolated from direct connection with the outer air ,and can only be supplied with air admitted through the duct 8, the connecting duct 20 and the space surrounding the upper inlet and dened by parts 50,5Iand2ll, Y

By the instrumentalities hereinabove described, it will be obvious that outflow from the iirebox will be constrained to pass through the duct 8, which will become closedbefore soot or the like It is customary to provide furnaceshaving oil, gas or analogous burners with a. high-low 'flame control. This control functions to turn the ame down in response to predetermined temperatures oi'Y pressures in the boiler. A conventional showing'of such a control is designated 22 in Figs. 1 and 2, wherein the weighted arm 23 rises in re'- spouse to a predetermined temperature or pressure inthe boiler and the Vconsequent pull on the chain Nlifts the weighted arm 25 partially to close the primary air'control valve 28 in the burner 5 and partially to close the oil supply valve 21 which is operatedthrough some such linkagesystem 'asjthat showny in Flgs..1 and 2. Itis desirable, when the high-low flame control moves to reduce the llame, correspondingly to reduce the input rate of secondary air. My invention is 'admirably adapted to be used in cooperation with a high-low flame control and an illustrative embodiment of one manner of effecting such cooperation is shown in Figs. 1 and 2'. The primary air control valve rod 25 is extended and its free end pulls a'depending chain 28 which elevates and lowersv a weighted rod 29 fast, on the end of aA rotatable' rod 30 which extends through the mounting plate I2 for the shutters ii and carries at its inner end a specially formed double cam 3i, 'surfaces of which engage the shutters Il. The double cam may take the form of a straight rod set at an inclination to the vertical, or it .may be a rod having opposite angularly directed ends. In any event the arrangement is such that rotation of the rod 30 tends to spread the shutters Il outwardly. It will be noted thata particular advantage of the arrangement described vresides in the `fact that the shutters Il are moved toward closed position'by rotation `of the rod 30 in either direction, so that the rod 29 and chain 28 may be disposed on` the right-hand 4side or the left-hand side of the rod`30. In this way the device may be incorporated in different furnace installations regardless of the location of the high-low flame control.

It will be appreciated thatwhen the high-low flame control moves to reduce the flame size the shutters Il are correspondingly moved toward closed position to constrict the opening through duct 8 and correspondingly reduce the rate of admission of secondary air.

From the foregoing explanation it is evident that the invention contemplates simple and efy for a furnace,

fective meansto attain the objects specified in the introduction to the specification. It is to be understood that the construction described has been selected merely to exemplify the invention and not to limit it to the details shown. 'nie invention is capable of embodiment in other and modined forms. and all such modifications. to the extent that they incorporte'the principles of the invention as defined by the appended claims, are to be deemed within the scope and purview thereof. u

I claim: l

1. In an air admission and control apparatus for a furnace, a duct adapted to convey combustion air to the furnace.. and closure meansin said duct normally open and' adapted to close upon the occurrence ofreverse flow of gases through the duct, said closure means comprising Ashutter means relatively near the inlet end of the duct movable to close the duct and sec-A ond shutter means relatively near the discharge end o! the duct movable by said reversely nowing gases partially to close the duct to direct said gases against .the nrst named shutter means to close the same. Y

2. In an air admission 'and control apparatus for a furnace, a duct adapted to convey combustion air to the furnace, and closure means in said duct normally open and adapted to close upon the occurrence of reverse flow of gases through the duct, said closurev means comprising a pair of shutters hinged in the duct relatively of shutters being movable by said reversely ow ing gases partiallylto close the duct to direct said gases between the spaced axes and against the first named shutters to close the same.

3. In an air admission and control apparatus for a furnace, a duct adapted to convey combustion air to the furnace, and closure means in said duct normally open and adapted to close upon the occurrence of reverse ilow of gases through the duct, said closure means comprising shutter means relatively near the inlet end of the duct, biased to open position and movable to close the duct, and second shutter means re1- atively near the discharge end of the duct, biased to open position and movable by said reversely owing gases partially to close the duct to direct said reversely flowing gases against the rst named shutter means to close the same.

4. In van air admission and control apparatus a duct adapted to convey combustion air to the furnace. closure means in said duct normally open and adapted to close upon the occurrence inthe duct of reverse flow of gases due to pressures withina predetermined range, said v'closure means comprising shutter means relatively near `the inlet end of the duct ymovable to close the duct. second shutter means relatively near the discharge end of the duct movable by said reversely flowing gases partially to close the duct to direct said gases against the first named shutter means to close the same, and safety means in the i'lrst named shutter means openable in response to reverse flow due to prres-l sures above those in said range to ope'n the duct.

5. In an air admission and control apparatus for a furnace, a duct adapted to convey combustion air to the furnace, closure means in said duct normally open and adapted to clo upon the occurrence in the duct of reverse ow of gases due to pressures within a predetermined range, said closure means comprising shutter means relatively near the inlet end of the duct movable to close the duct, second shutter means relatively near the discharge end of the duct movable by said reversely flowing gases partially to close the duct to direct said gases against the first named shutter means to close the same, and safety means in each of the shutter means openable in response to reverse flow due to pressures above those in said range to open the duct.

6. In an air admission and control apparatus for a furnace, a duct adapted to convey combustion air to the furnace, closure means in said duct normally -open and adapted to close upon' o the occurrence in the duct of reverse flow of gases through the duct, said closure means comprising shutter means relatively near the inlet end of the duct movable to close the duct, second shutter means relatively near the discharge end of the duct movable by said reversely flowing gases partially to close the duct to direct said gases against the first named shutter means to close the same, andadiustable stop means associated with at least one of said shutter means for determining the open position thereof.

7. In an air admission and control apparatus for a furnace, a duct adapted to convey combustion air to, the furnace, closure means in said duct normally open and adapted to close upon the occurrence in the duct of reverse fiow of gases through the duct, said closure means comprising shutter means relatively near the inlet end of the duct movable to'close the duct, second shutter means relatively near the discharge end of the duct movable by said reversely flowing gases partially to close the duct to direct ,said gases against the first named shutter means to close the same, and adjustable stop means associated with each of said shutter means for determining the open position thereof and the air capacity of the duct.

8. In an air admission and control apparatus for a furnace, a duct adapted to convey combustion air to the furnace, closure means in said duct normally open and adapted to close upon the occurrence in the duct of reverse flow of gases due to pressures within a predetermined range, said closure means comprising shutter means relatively near the inlet end of the duct movable to close the duct, and second shutter means relatively near the discharge end of the duct movable by said reversely flowing gases partially to close the duct to direct said gases against the first named shutter means to close the same, the rst named shutter means comprising a door frame and a door normally engaged therewith, said frame and'door being normally movable together, said door being movable away from the door frame to open the duct in response to reverse flow due to pressure above those in said range.

9. In an air admission and control apparatus for a furnace, a duct adapted to convey combustion air to the furnace, closure means in said duct normally open and adapted to close upon the occurrence in the duct of reverse ow of gases due to pressures within a predetermined range, said closure means comprising shutter means relatively near the inlet end of the duct movable to close the duct, and second shutter means relatively near the discharge end of the duct movable by said reversely flowing gases partially to close the duct to direct said gases against the first named shutter means to close the same, each of said shutter means comprising a door frame and a door normally engaged therewith, said frame and door being normally movable together, said door being movable away from the door frame to open the duct in response to reverse flow due to pressures above those in said range.

10. In a furnace, a burner, high-low flame control means therefor, a duct adapted to convey secondary air to the furnace, closure means in said duct normally open and adapted to close upon the occurrence of reverse flow of gases through the duct, said closure means comprising shutter means relatively near the inlet end of the duct movable to close the duct and second shutter` means relatively near the discharge end of the duct movable by said reversely flowing gases partially to close the duct to direct said gases against the first named shutter means to close the same, and means engageable with said closure means and responsive to said high-low flame control means for regulating the open position of said closure means.

11. In a furnace, a burner, high-low ame control means therefor, a duct adapted to convey secondary air to the furnace, closure means in said duct normally open and adapted to close upon the occurrence of reverse flow of gases through the duct, said closure means comprising shutter means relatively near the inlet end of the duct movable to close the duct and 'second shutter means relatively near the discharge end of the duct movable by said reversely flowing gases partially to close the duct to direct said gases against the first named shutter means to close the same, and means engageable with the first named shutter meansV and responsive to said high-low flame control means for regulating the open position of said first named shutter means.

12. In a furnace, a burner, high-low flame control means therefor, a duct adapted to convey secondary air to the furnace, closure means in said duct normally open and adapted to close upon the occurrence of reverse flow of gases through the duct, said closure means comprising a pair of shutters hiriged in the duct relatively near its inlet end and movable to close the duct and second shutter means relatively near the discharge end of the duct movably by said reversely flowing gases partially to close the duct to direct said gases against the first named shutter means to close the same, and means for regulating the open position of said closure means comprising a member pivoted between said pair of shutters and movable in response to said high-low ame control means to engage said shutters.

PHILIP O. HENDERSON. 

